The use of volatile organic compounds as carriers for the delivery of coatings is well-known. However, increasingly there is a need for environmentally friendly carriers which minimize the use of organic carriers such as organic solvents. Supercritical fluids have emerged as such a viable carrier in coating applications, particularly in applications requiring the delivery of a substance in spray form. While supercritical fluids are known to have solvating powers similar to organic solvents, they also present advantages over organic solvents because of their higher diffusivities, lower viscosities, and lower surface tensions.
A supercritical carrier may be considered any compound at a temperature and pressure above certain critical values of temperature and pressure. The critical temperature of a compound is the temperature above which the pure compound in gaseous state cannot be converted to a liquid, while a compound's critical pressure is the vapor pressure of the pure compound in gaseous state at the critical temperature. The critical point of the compound occurs at the temperature and pressure at which the gas and liquid phases are no longer separately defined, but instead a fluid exists in a state that is considered neither liquid nor gas. In the supercritical state, a fluid confers the carrier properties expected from a liquid while at the same time providing transport characteristics expected from gases.
Various compounds are known to exist as supercritical fluids, including ethylene, carbon dioxide, ethane, nitrous oxide, propane, and even methanol and water. The low cost and ready availability of supercritical carbon dioxide have made it a popular choice for a variety of applications. Also, with its critical temperature of 31.1° C., critical pressure of about 73 atm, and critical density of about 470 kg/m3, supercritical carbon dioxide has properties amenable to applications using standard pressure vessel technology.
Various applications have been explored for supercritical carriers, including use in the delivery of protective coatings to various commercial building substrates such as marble, stone, cast stone, architectural terra cotta, concrete, and concrete block. The degradation of such materials due to pollution, acid rain, and other destructive forces can be substantially decreased if a relatively thin protective coating is applied.
Several supercritical fluid technologies have been disclosed by investigators. For example, U.S. Pat. No. 4,923,720 to Lee et al. is directed to the use of supercritical fluids as diluents in the liquid spray application of coatings. A process and apparatus for coating substrates is provided in which a supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations.
However, prior art methods and devices for applying coatings using supercritical fluids suffer from complexity and concomitant bulky equipment, rendering the technology inconvenient to use and inaccessible to many potential customers. Commercial and laboratory equipment for applying coatings using supercritical fluids generally fall into two classes, batch and continuous. Typically, the main storage element of prior art batch systems is a floating piston accumulator. The coating material and supercritical fluid are held captive on one side of the piston, while the pressurization fluid is stored on the other. In such systems, the coating material and CO2 are added at a pressure typically above 1000 psi so that the CO2 remains in a liquid state. Such an arrangement requires high-pressure pumps. After the desired amounts of coating material and CO2 have been added, the two components must be mixed. Mixing usually is effected by circulating material in and out of the piston accumulator. The pressurizing fluid, disposed on the other side of the piston accumulator, is used to effect transport of the deliverable substance through a hose to a spray nozzle. Such batch systems are heavy due to the weight of the piston accumulator, high-pressure pumps, and associated controls. The weight of commercial units ranges between about 3000 lbs and about 1500 lbs. for equipment capable of delivering 6 kgs per batch, not including the CO2 supply bottle.
Continuous systems typically require two or three high-pressure pumps, along with complex flow meters and controls for accurately metering and mixing the coating material and supercritical fluid components. Multiple control loops and a programmable logic controller may be required. Such systems are less common, due to the required level of sophistication of controls. Further, although the commercial, continuous systems are capable of supplying about 100 grams to about 300 grams per minute of deliverable product, they are heavy, typically weighing between about 180 lbs. and 1000 lbs.
The above-described batch and continuous systems are heavy, bulky, require multiple high-pressure pumps, and require heavy CO2 cylinders with high stored energies. These systems also require significant equipment maintenance, as well as an additional energy source to power pumps and controls.
Thus, there exists a need for an improved apparatus and an improved method for dispensing coatings using supercritical fluids. There also exists a need for an apparatus with simplicity in design, compactness, and portability so that the device may be manually transported. Moreover, there exists a need for methods and devices that can deliver coatings with controllable composition and thickness.